The third generation partnership project (3GPP) specification release 10 has introduced carrier aggregation (CA) that bundles multiple component carriers for data transmission in order to actualize wideband radio communication over 20 MHz (e.g., at 100 MHz spectrum). See, for example, 3GPP TS36.300, v11.3.0. This technique has a limitation that component carriers are bundled only among cells under a same base station or evolved node B (eNB).
On 3GPP release 12 and the subsequent releases, inter-site CA has been discussed to bundle cells under different base stations (or eNBs) for data transmission. Inter-site CA is one of expedients to achieve more flexible network architectures.
FIG. 1 illustrates a proposed architecture for inter-site CA. For example, control plane (C-plane) data transmission (signaling radio bearer) that requires reliability is carried out from a macro cell under a macro eNB 20. On the other hand, user plane (U-plane) data transmission (data radio bearer) that requires wideband communication is carrier out from a small cell under a small eNB 30. In this case, it may be expected that the macro cell is configured as a primary serving cell (referred to as a “PCell”) that has high reliability, and that the small cell is configured as a secondary serving cell (referred to as a “SCell”) which is an additional cell.
By introducing a small cell into a macro cell, a part of data traffic of user equipment 10 can be unloaded to the small cell. Under the situation where C-plane data and U-plane data are separated, an architecture in which the macro cell secures connectivity and mobility for the user equipment 10 using a lower frequency band, while the small cell uses a higher frequency band to improve the throughput, may be conceived.